The present disclosure is directed to control methods for mixing and pumping systems, and more particularly, but not by way of limitation, to control methods for well service fluids, well cement preparation, and well fluid delivery systems used in hydrocarbon well bore servicing applications.
A control system typically comprises one or more physical system components employing a logic circuit that cooperate to achieve a set of common process results. In a mixing and pumping operation, the objectives can be to achieve a particular composition and flow rate for the resulting mixture.
The control system can be designed to reliably control the physical system components in the presence of external disturbances, variations among physical components due to manufacturing tolerances, and changes in inputted set-point values for controlled output values. Control systems usually have at least one measuring device, which provides a reading of a process variable, which can be fed to a controller, which then can provide a control signal to an actuator, which then drives a final control element acting on, for example, a flow stream. Examples of a of final control elements include flow control valves and speed controlled pumps.
The control system can be designed to remain stable and avoid oscillations within a range of specific operating conditions. A well-designed control system can significantly reduce the need for human intervention, even during upset conditions in an operating process.
In a hydrocarbon well bore servicing process, a control system can be used when mixing materials to achieve a desired mixture composition and flow rate. For example, when drilling an oil or gas well, it is common to install a tubular casing into the well bore and to cement the casing in place against the well bore wall. A cement mixing system that supports well bore servicing operations can be designed with a control system configured to provide a desired volumetric or mass flow rate of mixed cement having a desired density or composition in order to achieve desired properties of the cured cement. In particular, the cement mixing control system can control valves to regulate the in-flow of dry cement material and water into a mixing system to obtain the desired cement mixture density and desired cement mixture volumetric or mass flow rate out of the mixing system. The control system can operate, for example, by monitoring the cement mixture flow rate and density, and by regulating an in-flow water control valve and an in-flow dry cement material control valve. But sometimes, the amount of instrumentation available at well service sites is limited. For example, a water flow meter can be routinely present because measuring devices such as turbine meters or Coriolis mass flow meters are reliable and easy to maintain. However, solids flow meters, such as a weigh belt feeder, are much more difficult to service and to keep in calibration. Thus, such solids flow measuring devices are often not present. So, the control system is faced with a challenge as to how to monitor flow rates of a powdered solid, such as cement, without actually measuring the flow rate.
During a well bore cementing operation, the mixed cement is pumped down-hole at a target rate. Sometimes, supply of a particular component can get interrupted momentarily or constrained somehow (e.g. a supply constraint), and can cause a control disturbance to an automatic control system controlling the supply valve actuators and pumping system. For example, dry cement can be supplied from unitized storage systems, e.g. “pods”, that require change-over when they become empty, and thus, momentary interruption of the supply of dry cement can occur. For another example, the flow rate of a particular material can be unintentionally and/or unknowingly restricted due to a partial blockage of a supply line. For example, dry cement can pick-up moisture and begin to coat the interior of pipes, or collect at conduit elbows or valves, resulting in a restriction.
One skilled in the art of hydrocarbon well serving can appreciate the volume and speed at which well service fluids are prepared and pumped down-hole in a substantially time-sensitive manner with little or no chance to correct an error, since, for example, a slug of defectively-mixed cement can end-up a mile or more underground. When a supply interruption or constraint occurs, the control system can be faced with a challenge, especially when limited flow rate information is available, as to how to best react to balance quality control of the service fluid, e.g. density control, and the required supply rate, e.g. barrels per minute of fluid demanded down-hole.
Accordingly, a need exists for a mixing control system and a mixing control method that partially couples the effects of changes in the supply availability of the materials to be mixed with the desired supply rate and desired quality of the final mixture.